Abstract
Commercial grade-1 titanium samples (Ti, 99.6%) were treated using three alternative methods, (i) femtosecond laser processing, (ii) thermal heat treatment, and (iii) electrochemical anodization, respectively, resulting in the formation of differently conditioned superficial titanium oxide layers. The laser processing (i) was carried out by a Ti:sapphire laser (pulse duration 30 fs, central wavelength 790 nm, pulse repetition rate 1 kHz) in a regime of generating laser-induced periodic surface structures (LIPSS). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning setup for the processing of several square-millimeters large surface areas covered homogeneously by these nanostructures. The differently oxidized titanium surfaces were characterized by optical microscopy, micro Raman spectroscopy, variable angle spectroscopic ellipsometry, and instrumented indentation testing. The tribological performance was characterized in the regime of mixed friction by reciprocating sliding tests against a sphere of hardened steel in fully formulated engine oil as lubricant. The specific tribological performance of the differently treated surfaces is discussed with respect to possible physical and chemical mechanisms.
Highlights
Titanium is a technologically relevant metal with high corrosion and temperature resistance, good biocompatibility, and an extraordinary strength-to-weight ratio
The smoothest surface is observed for the POLISHED sample, while pronounced laser-induced periodic surface structures (LIPSS) with periods between 500 and 600 nm are present at the surface of the laser oxidized one (LASOX) sample
While polarization anisotropy is caused by optical scattering and diffraction at the sub-wavelength sized grating-like LSFL surface topography, the ellipsometric data suggest that depth graded oxide layers without sharp interfaces were formed upon fs-laser irradiation
Summary
Titanium is a technologically relevant metal with high corrosion and temperature resistance, good biocompatibility, and an extraordinary strength-to-weight ratio. These properties render it attractive for industrial, aeronautical, or medical applications [1]. In previous publications [2,3,4] we already reported the beneficial effect of the femtosecond (fs) laser processing of titanium surfaces (compared to the polished reference surface) for tribological applications. We perform additional experiments to elucidate the relevance of superficial oxidation on the tribological performance of titanium surfaces by comparing two traditional ways of surface engineering via thermal and anodic oxidation with the fs-laser processing method. The impact of the oxide layer thickness, its material structure and the superficial hardness are related to the current knowledge of tribological mechanisms acting for lubricants with additives
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